Mobile phones, personal digital assistants (“PDAs”), digital cameras, MP3 players, and other electronic devices utilize light-emitting diodes (“LEDs”), organic light-emitting diodes (“OLEDs”), polymer light-emitting diodes (“PLEDs”), and other solid-state transducer devices for backlighting. Solid-state transducer devices are also used for signage, indoor lighting, outdoor lighting, and other types of general illumination. FIG. 1A is a cross-sectional view of a conventional LED die 10a with lateral contacts. As shown in FIG. 1A, the LED die 10a includes a substrate 20 carrying an LED structure 11 having an active region 14, e.g., containing gallium nitride/indium gallium nitride (GaN/InGaN) multiple quantum wells (“MQWs”), positioned between N-type GaN 15 and P-type GaN 16. The LED die 10a also includes a first contact 17 on the P-type GaN 16 and a second contact 19 on the N-type GaN 15. The first contact 17 typically includes a transparent and conductive material (e.g., indium tin oxide (“ITO”)) to allow light to escape from the LED structure 11.
FIG. 1B is a cross-sectional view of another conventional LED die 10b in which the first and second contacts 17 and 19 are opposite each other, e.g., in a vertical rather than lateral configuration. During formation of the LED die 10b, the N-type GaN 15, the active region 14 and the P-type GaN 16 are stacked sequentially on a growth substrate (not shown), similar to the substrate 20 shown in FIG. 1A. The first contact 17 is formed on the P-type GaN 16, and a carrier substrate 21 is attached to the first contact 17. The growth substrate is then removed and the second contact 19 is formed on the N-type GaN 15. The structure is then inverted to produce the orientation shown in FIG. 1B.
Many LED devices (e.g., including the LED dies 10a and 10b of FIGS. 1A and 1B) are operated using AC power rather than direct current (“DC”) power because it reduces the complexity of the power supply electronics and, accordingly, can reduce the overall cost of the device (e.g., by removing the AC/DC converter). Typical AC-driven LED devices include LED dies (e.g., the LED dies 10a and 10b) coupled together in series and connected directly to an AC power supply. This configuration, known as “basic direct AC”, has a low overall efficiency because the individual LEDs produce virtually no light except for when the AC voltage is near its peak value. That is, the LEDs only operate when there is sufficient voltage to power all of the LEDs in the device. Other AC-driven LED devices include a variable length chain of LED dies, in which one or more LEDs are switched on as the voltage increases. This configuration, known as “multi-tap direct AC”, provides greater overall power utilization than basic direct AC systems because at least a portion of the LEDs are driven at lower currents.
Both the basic direct and multi-tap LED device arrangements typically include a large number of LED dies. However, for certain applications such large numbers of LED dies render the use of AC power infeasible. For example, the total emitter area (i.e., the area of a device provided for the light emitting source) in spot lighting is typically constrained to such a small area relative to the overall optics that often times only a single LED die is used. Achieving the same lumen density with the assembly of LED dies used in conventional AC power approaches would therefore be unfeasible.